Project Summary Lymphatic malformations (LMs) are the most common head and neck vascular malformation, affecting approximately 1-in-4000 live births. They can be significantly disfiguring and impair critical functions of the head and neck as they grow. An emerging body of evidence supports the contribution of postzygotic (mosaic) mutations in PIK3CA to the development of LMs. We propose to further characterize the mutation spectrum of lymphatic malformations and correlate the genetic findings to clinical phenotypic data. This will be performed with targeted deep sequencing technologies (smMIPs and ddPCR) to accurately detect mutations and sensitively measure the level of mutated cells present within the LM tissue. This will be performed on the largest cohort of LM patients studied to date (~88 patients) with tissue previously obtained from clinically indicated surgical resection. The genetic findings, including site of mutation and alternate allele frequency (AAF), will be correlated to phenotypic data including surgical stage, presence of lymphocytopenia, radiographic appearance, and size on volumetric imaging. We will also prospectively collect multiple samples from unique LMs and perform deep targeted sequencing to create mutation gradient maps based on preoperative imaging. We hypothesize that PIK3CA mutations will be detected in ~90% of LMs and that increased phenotypic severity (i.e. increased size, macrocystic radiographic appearance, lymphocytopenia, higher surgical stage) will be associated with hotspot mutations (mutations in PIK3CA previously observed in cancer) and increased AAF. We also hypothesize that AAF will be highest at the center of the lesions, lower near the edges of the lesions, and will not be present in surrounding ?normal? or hypertrophied tissue. Our goal is to better understand the etiology of LMs to inform future research regarding cell non-autonomous effects of mutated cells, options for molecular diagnosis, and future targeted therapies. This project will be done under the mentorship of Dr. William Dobyns, Dr. Jonathan Perkins, and Dr. James Bennett as part of a structured training program that includes formal coursework in genetics and statistics and training in ethics and grant writing.